1. Field of the Invention
This invention relates to an improved method and apparatus for drilling a well from a floating vessel. More particularly, the invention relates to a method and apparatus for maintaining a controlled hydrostatic pressure in a drilling riser.
2. Description of the Prior Art
In recent years the search for offshore deposits of crude oil and natural gas has been extended into deep waters overlying continental shelves. With increased water depths, the conduct of drilling operations from floating vessels has become more prevalent since economic considerations militate against the use of bottom-founded drilling platforms commonly used in shallow water. The drill rig and associated equipment are positioned aboard a floating vessel which is stationed over the wellsite. The drill string extends from the vessel to the bottom of the well through a wellhead on the floor of the body of water. A separate return conduit, normally a riser pipe, is positioned between the wellhead and the vessel to return the drilling fluid to the vessel.
Well control is an important aspect of any drilling operation. Conventionally, well control is established by maintaining the density of the drilling fluid and thus the hydrostatic pressure exerted on the subsurface formations at a level sufficient to overcome formation pressures and to prevent the influx of fluid from the formation. If uncontrolled, influx of formation fluids can lead to a blowout and fire, frequently causing loss of life, damage to property, and pollution of the seaway.
At the same time, caution is necessary to assure that the density and resulting pressure gradient of the column of drilling fluid does not exceed the natural fracture gradient of the formation, i.e., the pressure gradient necessary to initiate and propagate a fracture in the formation. This consideration is especially important in deep waters where the natural fracture gradient of shallow formations does not greatly exceed that of a column of sea water. To minimize the possiblity of formation fracture during the drilling of the surface hole (the first few thousand feet), the hydrostatic head of the drilling fluid should not greatly exceed that of a column of sea water.
In deep water, achieving a hydrostatic head high enough to control the well and yet low enough to prevent fracturing subsurface formations requires careful control of the pressure gradient of the drilling fluid. Where the drilling traverses two different formations having different pressures, it is possible that the hydrostatic pressure of the drilling fluids in the return column from the drill bit can become great enough to enter into one formation (a condition known as "lost circulation"). With lost circulation, the hydrostatic pressure of the drilling fluid can decrease below the pressure of the other formation and cause a blowout.
If the mud returns are conducted to the ship through the riser, the pressure of the mud which acts on the formation is a function of the density of the mud returns. When drilling shallow formations the drill cuttings can cause a sufficient mud density increase to cause formation fracture. Previously proposed solutions for this problem have involved decreasing the density of the drilling returns, either by decreasing the rate of penetration, increasing the circulation rate of the input fluid, or injecting gas into the riser. Each approach is subject to certain difficulties. Decreasing the penetration rate requires additional rig time with the attendant expense. Increasing the circulation rate frequently requires extra pumps and may lead to erosion of the borehole. Gas injection requires assembly and deployment of additional complex and expensive equipment at the well site.